Conventional FM broadcast radio signals are transmitted according to a standardized scheme. Mono FM broadcast signals are modulated by an audio signal with 15 kHz bandwidth. This is known as the M signal. The modulating signal of stereo FM broadcasts additionally contains a pilot signal at 19 kHz and a second signal representing the difference between the left and right stereo channels which is transmitted as a double-sideband suppressed carrier (DSBSC) signal in the range from 23 kHz to 53 kHz. This second signal is known as the S signal. In generating stereo signals for transmission, the M, S and pilot signals are combined producing a signal known as the stereo multiplex, and then frequency modulated. Signals carrying other information, such as RDS, RBDS, ARI and subsidiary communications may also be added to a stereo multiplex. For mono reception only the M signal needs to be received. For stereo reception the left and right stereo channels are recovered by combining the M and S signals. The pilot signal aids the receiver to perform separation of the stereo multiplex components.
When an FM signal is to be received and decoded for reproduction, it is frequency demodulated to yield the stereo multiplex signal. If the FM signal is transmitted as a stereo signal then the stereo multiplex contains the M, S and pilot signals. If the FM signal is transmitted as a mono signal then the stereo multiplex contains the M signal but it does not contain the S and pilot signals. Nowadays most FM radio stations transmit stereo broadcasts. The structure of the stereo multiplex for a stereo FM broadcast signal is illustrated in FIG. 1.
For various reasons it might be desirable for the receiver to have a measure of the quality of the received signal. For example, when a single radio station is broadcast on two FM channels it may be useful for the receiver to switch automatically to the channel that can be received with better quality. To achieve this, the receiver must be able to compare the receive quality of those two channels. Another use for a quality measure is to provide the input to a quality meter for providing an indication of receive quality to a user of the receiver.
A conventional way to assess received FM signal quality is by taking received signal strength as a measure of received signal quality. However, this approach can not detect signal impairments that do not affect the signal's strength, such as interference and distortion.
There is therefore a need for an improved method for estimating the signal quality of received FM radio signals.